[Illustration: FIG. 166.--Sound is carried by the air.]
Since the sound of the bell grows fainter as air is removed, we infer
that there would be no sound if all the air were removed from the
flask; that is to say, sound cannot be transmitted through empty space
or a vacuum. If sound is to reach our ears, it must be through the
agency of matter, such as wood, water, or air, etc.
252. How Sound is transmitted through Air. We saw in Section 250
that sound can always be traced to the motion or vibration of matter.
It is impossible to conceive of an object being set into sudden and
continued motion without disturbing the air immediately surrounding
it. A sounding body always disturbs and throws into vibration the air
around it, and the air particles which receive motion from a sounding
body transmit their motion to neighboring particles, these in turn to
the next adjacent particles, and so on until the motion has traveled
to very great distances. The manner in which vibratory motion is
transmitted by the atmosphere must be unusual in character, since no
motion of the air is apparent, and since in the stillness of night
when "not a breath of air" is stirring, the shriek of a railroad
whistle miles distant may be heard with perfect clearness. Moreover,
the most delicate notes of a violin can be heard in the remotest
corners of a concert hall, when not the slightest motion of the air
can be seen or felt.
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